JP2006255771A - Method and apparatus for cooling steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for cooling a steel tube after being rolled in a seamless steel tube production process, and, more concretely, to provide a technique of preventing rake cracks generated in a seamless steel tube with a relatively small diameter such as a 9Cr steel tube. <P>SOLUTION: The cooling apparatus is used for a production apparatus of a seamless steel tube such as a 9Cr steel tube, wherein in a cooling bed 10 for transporting a steel tube, the upper face of the cooling bed 10 in a fixed section on the upstream side is provided with each heat insulating material layer 20 composed of a ceramic fiber spun material reinforced by a stainless steel reinforcing wire, so that the thermal conduction is prevented in each part at which the steel tube transported by stepping in the cooling bed is contacted with the cooling bed 10. For example, such a structure is employed that the upper face of a cooling bed hardware 10 is provided with a recessed hole 13, and the leg part 22 of each fitting body 21 whose upper face is loaded with the heat insulating material 20 is inserted into the recessed hole 13. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for cooling a steel pipe after rolling in a seamless steel pipe manufacturing process and an apparatus therefor. More specifically, the present invention relates to a technique for preventing rake cracks that occur in a relatively small diameter seamless steel pipe, such as a 9Cr steel pipe.

  Small-diameter seamless steel pipes with an outer diameter of 120 mm or less are heated after pipe material (round billet), pierced with a piercer, piped through a mandrel mill and a reducer, and then subjected to heat treatment such as quenching, tempering, and normalizing. After sizing with a mill, various tests, threading, etc. are performed to make a product.

  In this steel pipe manufacturing process, the seamless steel pipe is gradually cooled while being conveyed on the cooling bed after being formed by a reducer or the like. In this cooling, for example, in a steel type having a high cracking sensitivity such as a 9Cr steel pipe having a Cr content of 8.0 to 9.5% by mass, in the vicinity of the portion of the cooling bed in contact with the moving bed, a heat treatment step of the next step is performed. There is a problem of generating cracks called so-called rake cracks. Various causes have been investigated for such rake cracks.

  For example, reduction of the quenching index by improving the quality of the steel material, reduction of N to reduce the N content in the steel, and the like are performed. Even with such measures, conventionally, rake cracks could not be completely prevented.

  The inventor of the present invention pays attention to the fact that rake cracks often occur in the cold season of the year, and as a result of investigating the cause after repeated research, as one of the causes of the rake crack generation mechanism, The new knowledge that it originated in the difference generation of the thermal expansion coefficient accompanying the transformation of the steel structure caused by the local cooling of the steel pipe in the contact part of the steel pipe and the cooling bed was obtained. For this reason, it was estimated that a subtle difference in local cooling due to a difference in environmental temperature appeared between the cold season and the warm season in the year, resulting in a difference.

  An object of the present invention is to develop and provide an effective countermeasure plan for preventing the occurrence of a difference in thermal expansion coefficient due to local cooling based on such knowledge.

  The present invention has been made to solve the above problems, and is a steel pipe cooling method characterized by taking the following technical means. That is, this invention is a cooling method of the steel pipe characterized by preventing that the upper surface of a conveyance stand locally cools the outer surface of a steel pipe in the cooling stage of a seamless steel pipe manufacturing process.

  In the present invention, the region for preventing the upper surface of the conveyance table from locally cooling the steel pipe may be a region where the steel pipe temperature is 750 ° C. or higher. The steel pipe temperature after processing by a reducer or the like is about 900 to 950 ° C., and the processed steel pipe is gradually cooled while being conveyed by a cooling device. The portion of the steel pipe not in contact with the cooling bed has a ferrite structure on the outer surface and a martensite structure on the inside of the wall thickness.

  The inventor has found that the portion in contact with the cooling bed is rapidly cooled by the cooling bed, and the structure of the steel plate in this portion becomes a martensite structure due to transformation, and rake cracks occur during heating in the next heat treatment stage. The main object of the present invention is to keep the surface of the steel pipe as a ferrite structure in the cooling process of the steel pipe in the same manner as the portion of the cooling bed that does not contact the cooling bed.

  The apparatus of the present invention capable of suitably carrying out the above-described method of the present invention is a steel pipe characterized in that, in a cooling apparatus for a seamless steel pipe manufacturing apparatus, a heat insulating material is attached to the upper surface of the cooling floor in a certain section on the upstream side of the steel pipe conveyance. The cooling device. Here, the constant section may be a section where the temperature of the steel pipe is 750 ° C. or higher. This can be determined according to the size, material, processing process, etc. of the steel pipe. If this section is determined for the steel type and process having the worst conditions, there is no problem in the better condition.

  In the steel pipe cooling device, the means for attaching the heat insulating material is not limited. However, the heat insulating material is attached to a mounting body in which a concave hole is provided in the upper surface of the cooling floor and the ceramic fiber textile material is attached to the upper surface. If the leg portion is inserted into the concave hole, maintenance management is easy, and the thermal insulation is preferably attached by detachably fixing the steel plate attached with the ceramic fiber textile material to the upper surface of the cooling floor. It is good. As the ceramic fiber textile material, for example, a material reinforced with a stainless steel reinforcing wire is preferably used.

  According to the present invention, it is possible to prevent local rapid cooling of the steel pipe by the cooling bed in an area where a partial structural change may occur in the cooling process of the seamless steel pipe manufacturing process. Can now be prevented. In addition, an effective effect has come to be obtained together with the improvement of the steel pipe material etc. which lowers the cracking sensitivity.

  When the compressive stress acts on the outer surface of the steel pipe due to the difference in thermal expansion coefficient between the inner and outer temperature of the steel pipe and the difference between the inner and outer structures during the cooling process after seamless steel pipe rolling, the outer surface will not crack. .

  In the high temperature region during the cooling process, in the portion where the steel pipe is in contact with the cooling bed, the structure in the vicinity of the outer surface of the steel pipe becomes a martensite structure by cooling. For this reason, the outer surface of a steel pipe and an internal thermal expansion coefficient become equal. A temperature difference occurs on the inner and outer surfaces of the steel pipe during heating such as heat treatment, and the stress generated on the outer surface of the steel pipe by this temperature difference becomes a tensile stress. As a result, the risk of cracking on the outer surface of the steel pipe increases.

  The temperature at which the structure changes varies depending on the steel type, but is generally in the range of 820 to 880 ° C. Therefore, it is preferable that the steel pipe after the rolling process is not rapidly cooled by the cooling bed in this region. In consideration of safety and variation, the steel pipe temperature range may be set to 750 ° C. or higher so that the steel pipe is not rapidly cooled by the cooling bed.

  Embodiments of the present invention will be described below with reference to the drawings.

  1 and 2 are explanatory views schematically showing a cooling bed in a steel pipe cooling process according to an embodiment of the present invention.

  In the cooling floor, a fixed bed and a moving bed in which triangular peaks and triangular valleys are connected in a saw-like shape in the longitudinal direction on the upper surface are adjacent to each other in parallel. As the moving bed repeats the cycle of ascending, advancing, descending, and retreating, the steel pipe on the cooling bed advances and advances on the fixed bed. The steel pipe is placed in contact with the triangular valley portion on the upper surface of the cooling bed, and the portion in contact with the cooling bed is further cooled by heat conduction to the cooling bed. This partial excess cooling causes rake cracking.

  Therefore, in order to prevent this, a heat insulating material is attached to the triangular valley portion in contact with the steel pipe of the cooling floor in the region where the steel pipe temperature is 750 ° C. or higher to prevent excessive cooling due to heat conduction.

  FIG. 1 is a perspective view of a cooling floor hardware 10 of the embodiment, and illustrates an example in which two continuous triangular mountains are provided. The cooling floor hardware 10 has a mounting portion 11 connected to a fixing or moving mechanism of the cooling floor. In FIG. 1, a concave hole 13 is provided on the upper surface 12 of the cooling floor hardware 10, and a leg portion 22 of a mounting body 21 with a heat insulating material 20 attached to the upper surface is inserted into the concave hole 13 as indicated by an arrow 23. An embodiment in which the heat insulating material 20 is mounted on the upper surface of the metal object 10 is shown.

As the heat insulating material 20, a ceramic fiber textile material reinforced with a stainless steel reinforcing wire was used. This ceramic fiber weaving material is made by weaving a heat insulating material mainly composed of Al ₂ O ₃ and SiO _{2. It} has a heat resistance of 1000 ° C. or higher when used continuously, and has a heat resistance of up to 1250 ° C. when used for a short time. It has excellent durability and high heat insulation.

  In the embodiment shown in FIG. 1, an example is shown in which a square hole is provided as the concave hole 13 and the leg portion 22 of the attachment body 21 to which the heat insulating material 20 is attached is inserted into the square hole. For example, the concave hole 13 may be a plurality of round holes or the like, and the shape of the leg portion 22 of the mounting body 21 may be a shape corresponding thereto.

  In FIG. 2, a groove-shaped steel plate 24 with a heat insulating material 20 attached to the upper surface is placed on the cooling floor metal 10, and the side wall 25 of the groove of the steel plate 24 is fixed to the side surface of the cooling floor metal 10 using fixing bolts 26. It is an example.

  Next, experimental examples showing the effects of the method of the present invention will be described. The composition is C: 0.08 to 0.12 mass%, Mn: 0.30 to 0.60 mass%, P: 0020 mass% or less, S: 0.010 mass% or less, Si: 0.20 to 0. A sample of 9Cr steel consisting of 50% by mass, Cr: 8.00 to 9.50% by mass, Mo: 0.85 to 1.05% by mass, and V: 0.18 to 0.25% by mass is shown in FIG. As shown in the temperature history curve 31, the structure was martensite when heated at a heating rate of 10 ° C. per second, held at 950 ° C. for 600 seconds, and then rapidly cooled. On the other hand, as shown in the temperature history curve 32 of FIG. 5, when heated at the same temperature rise rate, held at 950 ° C. for 600 seconds, then held at 750 ° C. for 1800 seconds, and then cooled, the structure was ferrite. It was.

  These two samples were heated by the temperature rising pattern 33 shown in FIG. 6, and the amount of thermal expansion was measured. The temperature rising pattern 33 in FIG. 6 simulates the heating pattern in the continuous annealing furnace. The measurement results of the thermal expansion amount are as shown in FIG.

  FIG. 3 is a graph showing thermal expansion amounts of ferrite and martensite in the reheating process. Curve 51 is the ferrite and curve 61 is the martensite thermal expansion curve. It is assumed that there is a temperature difference of 15 ° C. between the steel pipe outer surface temperature and the steel pipe inner surface temperature in the reheating process. When the inner and outer structures are ferrite, the difference in thermal expansion between the outer surface position 52 and the inner surface position 53 is small as shown by the thermal expansion difference 54. On the other hand, if the outer surface is martensite, the thermal expansion difference 63 between the outer surface position 62 and the inner surface position 53 becomes excessive, and a crack occurs at the outer surface position.

  The figure shows this. When the temperature difference between the inner and outer surfaces of the steel pipe is 15 ° C, the inner surface temperature is taken on the horizontal axis and the circumferential stress MPa on the outer surface is taken on the vertical axis, and the relationship between the two is shown. It is a thing. A curve 71 corresponds to the curve 61 in FIG. 3, and a curve 72 corresponds to the curve 51 in FIG. As the temperature changes, outer surface tension and outer surface compression occur around the inner surface temperature of 820 to 840 ° C.

  In curve 71, a peak circumferential tensile stress of about 130 MPa was generated. This value corresponds to the tensile strength of the γ phase at 850 ° C., and it was confirmed that rake cracking occurred.

It is a schematic explanatory drawing of the cooling bed of an Example. It is a schematic explanatory drawing of the cooling bed of another Example. It is a graph which shows the thermal expansion curve of a ferrite and a martensite. It is a temperature history curve of an experiment example. It is a temperature history curve of an experiment example. It is a temperature rising pattern of an experiment example. It is a graph of an estimated stress value.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Cooling floor hardware 11 Mounting part 12 Upper surface 13 Recessed hole 20 Heat insulating material 21 Mounting body 22 Leg part 23 Arrow 24 Steel plate 25 Side wall part 26 Fixing bolt 31, 32 Temperature history curve 33 Temperature rising pattern 51 Curve 52 Outer surface position 53 Inner surface Position 54 Thermal expansion difference 61 Curve 62 Outer surface position 63 Thermal expansion difference 71, 72 Curve

Claims (5)

  A method for cooling a steel pipe, characterized in that, in the cooling stage of the seamless steel pipe manufacturing process, the upper surface of the conveying table is prevented from locally cooling the outer surface of the steel pipe.   The method of cooling a steel pipe according to claim 1, wherein the seamless steel pipe is a 9Cr steel pipe.   In the cooling device of a seamless steel pipe manufacturing apparatus, a heat insulating material is mounted on the upper surface of the cooling floor in a certain section on the upstream side of the steel pipe conveyance.   4. The cooling of a steel pipe according to claim 3, wherein the mounting of the heat insulating material is provided with a concave hole on the upper surface of the cooling floor, and a leg portion of an attachment body attached with a ceramic fiber textile material on the upper surface is inserted into the concave hole. apparatus.   4. The steel pipe cooling apparatus according to claim 3, wherein the heat insulating material is attached by detachably fixing a steel plate attached with a ceramic fiber textile material to the upper surface of the cooling floor.

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